Microporous films are used in a wide range of applications, generally to provide a selective barrier. For example they may be used as battery separators, and electrolysis membranes as well as in breathable fabrics and medical and packaging applications. Commonly used polymeric films comprise polyolefins such as polyethylene and polypropylene which can conveniently be made porous by extraction of a soluble component. Such films are chemically inert towards many acids and alkalis and towards many reactive metals. However, there exist a number of solvents with which a polyolefin film cannot be used because of chemical incompatibility. Furthermore, the maximum operating temperature of polyolefin films is about 120.degree. C., and their use in high temperature applications is therefore not possible.
U.S. Pat. No. 3,859,402 (Bintliff) describes the preparation of a thin microporous fluorocarbon polymer sheet material alleged to have a uniform microporosity which was useful in preparing electrodes capable of breathing oxygen from air. Fluorocarbon polymer articles were mixed with particles of a metallic salt pore former, the resultant mixture was formed into a sheet material and the metallic pore former (which was e.g. calcium formate, sodium chloride or sodium carbonate) was removed e.g. by dipping the sheet into water. The polymer could be polytetrafluoroethylene, polytrifluoroethylene, polyvinylfluoride, polyvinylidene fluoride, polytrifluorochloroethylene and copolymers thereof.
U.S. Pat. No. 4,613,441 (Kohno et al, assigned to Asahi) describes a process for making a thermoplastic resin having a critical surface tension of not higher than 35 dyne/cm into a membrane having a three-dimensional network structure of intercommunicating pores. The network structure is contrasted with a through-pore structure in which pores extend substantially linearly through the membrane from the front surface to the back surface. The network structure including communicating pores has high porosity combined with long path length compared to a through-pore membrane of the same thickness and the actual pore diameter is much smaller than the diameter of the pores exposed on the surface. An initial porosity is formed in the membrane using finely divided silica which is dissolved in aqueous sodium hydroxide to give a structure having an average pore diameter of 0.05-1 micron and a porosity of 30-70%. The membrane is then stretched by space drawing in at least one direction to enhance the porosity and at the same time improve mechanical strength. In one example an ethylene/tetrafluoroethylene copolymer (Tefzel 200) is formed into a porous membrane of thickness 75 microns, average pore diameter of 0.55 microns and porosity of 85% with an air permeability of 60 sec./100 cc 100 microns measured by method A of ASTM D-762. However, the above ASTM test is done using mercury porosimetry and does not give a true picture of the interconnection between the pores of the material which governs air flow through it.
The resistance of the ethylene/tetrafluoroethylene copolymer (Tefzel) and the ethylene/chlorotrifluoroethylene copolymer (Halar) to the chemically adverse environment of a lithium battery is described in U.S. Pat. No. 4,405,694 (Goebel et al), but only in the context of an insulation sleeve of non-porous material for a conductive jumper element.